1. Field of the Invention.
This invention relates to thermal printing and more particularly to a system and method for automatically detecting any change in printhead resistance due to continued usage of the printhead and for automatically correcting for such resistance change in order to maintain constant printing energy.
2. Description of the Prior Art.
Many different types of thermal printers have been proposed for obtaining a substantially constant print quality or color density.
U.S. Pat. No. 4,113,391 discloses an apparatus for adjusting the pulse width of the pulses applied to printhead elements as a function of variations in supply voltage and ambient temperature.
U.S. Pat. No. 4,284,876 discloses a system which controls the pulse width of each pulse applied to a thermal element as a function of the moving speed of a thermal paper and/or the status (black/white) of the previously printed several dots so that the desired concentration or color density is obtained.
U.S. Pat. No. 4,391,535 discloses an apparatus for controlling the duty cycle or pulse width of a printing pulse for a thermal print element as a function of the estimated value of the temperature of that thermal print element.
U.S. Pat. No. 4,415,907 discloses a circuit which compares printing data for a present line with printing data for the preceding line which has already been printed, and decreases or increases the pulse widths of the printing pulses to the thermal resistor elements for the present line as a function of whether or not the elements in the print line were heated during the previous line.
U.S. Pat. No. 4,434,354 discloses a thermal printer which adjusts the pulse width as a function of the amplitude of a power supply voltage in order to maintain a constant record density.
None of the above-cited, prior art thermal printers adjusts the head voltage and/or pulse width of a printing pulse as a function of a change in the thermal printhead resistance. As a result, none of the above-cited, prior art thermal printers provides for compensating for resistance changes in the thermal printhead as a result of repeated use.